In general, this invention relates to a method and an apparatus for producing a controlled powder flow or discharge of powder and/or granular materials from a bulk storage facility. More specifically, the invention pertains to the use of hard permanent magnetic media materials subjected to an oscillating magnetic field to initiate and promote a controlled flow of non-magnetic powder material out of a storage facility, hopper, bin, or a feeder.
In most industrial applications requiring the handling and/or processing of granular materials or powders, these materials often need to be stored in containers such as hoppers, bins or silos, both before and after processing. There are many problems associated with the storage and discharge of these materials from the containers. In particular, there are serious problems with the discharge, which may be based on gravity flow or other means. These may include no flow condition, which usually occurs due to bridging, arching or ratholing. Factors such as humidity, bulk weight, powder size, powder cohesiveness, and heat may adversely affect the flow conditions. Arching can occur due to mechanical interlocking, or more frequently due to the cohesive strength of the solid. Ratholing generally occurs due to high friction between the hopper walls and the material and/or due to cohesion of the material under consolidation. In some cases, the theory of geometric hopper design of Jenike may be used to overcome no flow conditions. However, the minimum outlet dimension requirement imposed by the Jenike procedure is often unrealizable for the small batch sizes in high-value added applications. Moreover, these applications require devices that produce predictable and controllable amount of flows, and avoid segregation upon delivery. Therefore active discharge aids are required.
There are three categories of these devices: pneumatic, vibrational, and mechanical. In pneumatic methods, one can use aeration, air-blasts, or air-inflators. In aeration, a controlled amount of air is introduced in the bulk, at either a high-level or low-level of aeration. An example of this is Airsweep(trademark) by Myrlen, Inc., Boynton Beach, Fla., where powerful mini-pulses of air are directed between the material and the container wall to sweep and lift material off loping surfaces to promote flow. Another example is an air flow based Fluidizer, manufactured by Solimar Pneumatics, Minneapolis, Minn. In high-level aeration, the amount of air is sufficient to reduce wall friction and to make the material near the outlet aerated to flow like a liquid. The problem with this approach is that it may cause flooding.
In low-level aeration, the amount of air introduced is low, but enough to encounter de-aeration due to time consolidation. This prevents the flooding problem, but it cannot improve the situation where even the freshly introduced material has difficulty flowing out of the hopper. In air-blasting methods, for example, BIG BLASTER(copyright) Air Cannons manufactured by Martin Engineering, Neponset, Ill., a small quantity of high-pressure air is injected into the bulk in sudden bursts. This may impart shocks, which could break an arch or a rathole. There are potential disadvantages such as damage to the walls or the structure, and flooding.
Other pneumatic devices utilize inflatable pads mounted along the hopper walls, which are activated at regular time intervals. For highly cohesive materials, this technique can cause further problems due to increased compaction. In vibrational techniques, either the vibrations are applied to the hopper wall, or mounted vibrating devices agitate the entire powder bulk near the outlet, as in Geiser et al., U.S. Pat No. 5,472,117. Wall mounted vibrators may fail to transmit the vibrations where needed, and in some cases, further compact the material.
In methods such as Geiser et al. and others called bin-activators (manufactured for example by Vibra Screw Inc., Totowa, N.J.), a substantially impermeable pre-shaped member such as a plate or an inverted cone is supported inside the powder container, near its outlet. It is then coupled to a vibrator unit and upon the activation of vibrations, flow may occur. A disadvantage of this device is the need for providing and supporting the said member. Another disadvantage is that the diameter of the said member is preferred to be greater than or equal to the minimum arching or coring diameter of the powder material. This preference makes this device useless for small batch applications. Other potential disadvantage is the creation of noise when the device is operated.
In mechanical methods, an arm or a screw moves inside the bulk and it continuously acts against a stable formation of an arch or a rat-hole. For highly cohesive materials, excessive stresses are generated on the mechanical arm, making it susceptible to damage and high power needs. Overall, many of these methods may be effective for certain situations, for many cases hey do not work properly and even cause additional problems. These problems include uncontrollable flow, flooding, dust explosion, damage to hopper walls, segregation upon discharge, increased powder compaction, etc.
None of the prior art to actively promote powder flow out of a container such as a bin or a hopper is based on the use of small, dispersed media such as hard permanent magnetic particles. The prior art generally deals with excitations transmitted by one or a few large sources such as a vibrated member, compressed air unit(s), inflatable pads, or moving members. None are based on the idea of utilizing tens of thousands of individual excitation sources, such as individually propelled magnetic particles. Such magnetic media has been used in other applications listed in the following. In Kuznetsov et al., U.S. Pat. No. 3,987,967, magnetic elements of a magnetically hard material is placed in a chamber along with other material being worked upon. The action of an alternating magnetic field produced by an electric winding causes chaotic motion of the magnetic material and causes grinding of the powder material. In Hoffa, U.S. Pat. No. 4,090,263, one large magnetic element is rotated by the action of an externally positioned rotating magnetic bar to mix the sample materials such as biological fluids. Watanabe et al., in U.S. Pat. No. 4,632,316, describe a method and apparatus for electromagnetically processing substances by crushing, mixing and stirring the substances, including an apparatus equipped with a container for receiving ferromagnetic or non-magnetic conductive working pieces, together with substances to be processed by causing a shifting magnetic field to act on a generate a strong random motion of the working pieces. The emphasis, in contrast to Kuznetsov et al. is on properly changing the electromagnetic field intensity level. Abrosimov et al., in U.S. Pat. No. 4,995,730, disclose a method of electromagnetic working of materials wherein a stream of material to be worked is continuously fed into a working zone which accommodates a layer of magnetic elements, wherein a variable magnetic field is generated to actuate the layer of chaotically moving magnetic elements to work the material being fed. The material to be worked on can be a fluid, suspension or emulsion. The invention is to be used for dispersing, emulsifying and mixing suspensions, predominantly in the chemical and related industries. Abrosimov et al. invention is claimed to be an improvement of prior art (U.S. Pat. Nos. 3,219,318, 3,987,967) by improving the power efficiency of the process. Another improvement is described in Halbedel et al., U.S. Pat. No. 5,348,237, where an apparatus for reducing, dispersing, wetting and mixing pumpable, nonmagnetic, multiphase mixtures by means of magnetic media propelled by electromagnetic energy is described. This technique is limited to fluid based mixtures.
It is important to distinguish that the magnetic media is utilized in the prior art generally for working, dispersing, emulsifying, crushing, grinding, mixing, or coating the material of fluid, fluid-particle or particulate nature. None of the prior art is concerned with promoting a controlled amount of powder discharge out of a storage container using such magnetic media.
Related to the topic of powder flow from hoppers is the concept of an angle of repose (AOR) of the powder material. In general, this is the angle formed when a powder material is poured into a heap or a pile. In many powder and bulk material applications, one often needs to measure the AOR, because it is one of the indicators of the flowability of the material. There are several different ways to measure the angle of repose. In a tilt table method, the material for which the AOR is to be measured is placed evenly on a horizontal table or a platform. Then, the inclination of the table is gradually and slowly increased till the material just starts to flow. The angle at which this occurs is measured and is denoted as the AOR of that material. In another approach, the material for which the AOR is to be measured is poured to form a heap. The heap could be either of a fixed height or of a fixed base. The angle of that heap is then measured and is denoted as the AOR of that material. The pouring may be facilitated by use of one or more funnels or chutes. Another method utilizes a rotating cylinder in which the powder or bulk material for which the AOR is to be measured is placed. The sealed cylinder is then placed on a rotation station, such that its axis of revolution is also the axis of rotation. The speed of rotation is adjusted till more or less uniform, steady state slope of the powder or bulk material has formed. The angle of that slope is then measured and is denoted as the AOR of that material. There are other techniques such as in Soviet Union Patent No. 1,758,408, where AC (alternating current) at given frequency is passed to a solenoid of a vibro-drive, which applies vertical oscillating movement to a tank containing material to be tested. Some of the material falls through a hole in the bottom into a vessel. A conical concave surface is formed in the tank and a convex cone is formed in the vessel, whose angles correspond to the collapse and repose angles of the bulk material. The limitation of these methods in prior art is that it is difficult to get accurate, repeatable results for highly cohesive powders using these techniques. Moreover, none of the prior art utilizes magnetic particles as the media that creates controlled flow of the material to form a heap.
It is a general objective of the present invention to promote a flow of stored powder material from a bulk storage in a controlled fashion.
It is a related objective to provide a means to produce a metered discharge of powders at rates as low as a few grams or fractions of a gram per second.
It is an associated objective to provide a powder discharge technique that is particularly suited for small batch applications, involving about 1000 or 2000 Kg amounts.
It is a more detailed objective to provide a means to create evenly dispersed excitation within the powder bulk, generated by hundreds or thousands of individual sources such as the magnetic media.
It is an associated objective to minimize the segregation of the powder material upon discharge due to the chaotic nature of excitation of the powder.
It is a related objective to minimize adverse effects of powder excitation such as powder compaction or powder flooding.
It is another objective to minimize the need for internal support structures and large movable parts in the process of powder excitation.
It is yet another objective to have a device that minimizes the damage to the container due to powder excitation.
It is a related objective to reduce the level of noise associated with powder excitation.
It is an associated objective to have a device that produces stable, clean piles or heaps of powder and thus allow for accurate determination of angle of repose of the powder material.
It is an additional objective to have a simple device that minimizes the operator error during determination of angle of repose of the powder, and minimizes the occurrence of false peaks.
These and other objects of the invention which shall be hereafter apparent are achieved by a method for magnetically mediated controlled powder discharge which provides a means to promote a controlled flow of powder or dry bulk material out of a storage container such as a silo, bin or a hopper by the motion of magnetically hard permanent particles acting as the media within the powder near the storage outlet. The magnetic particles are excited by a sufficient electromagnetic energy supplied from an oscillating magnetic field. A screen or a mesh is placed near the outlet so as to prevent said magnetic particles from leaving the storage through outlet. Alternately, an additional, strong magnetic field that counteracts gravity is provided to keep said magnetic particles suspended within the container and thus are prevented from leaving the storage through outlet. Said additional magnetic field is placed in a suitable location and is suitably turned on and off to facilitate levitation of the magnetic particles. Action of individually excited magnetic particles create evenly dispersed excitation within the powder to promote an even flow and controlled discharge out of hopper.